Ion implantation is used for the incorporation of dopant impurities into semiconductor wafers for the formation of semiconductor devices such as field effect transistors (FETs) and bipolar junction transistors (BJTs). A focused beam of ions is generated in an apparatus called an ion implanter. The beam is produced by ionising molecules or atms in a source gas and then electrostatically collimating, filtering to exclude unwanted ions and focusing the beam on the target wafer surface. Ideally, the treated wafer surface has a uniform radial density and spot size generated by the focused beam. The entire wafer surface is exposed to the beam, and ions are driven into the wafer surface. The depth of how far the ion travels is dependent upon their energy within the beam. That is, ion density in the beam and time of exposure have great effect to the pattern of dosage implantation.
However, the actual density of ions is not uniform across the focused beam that forms the pattern. Typically, the dosage of the resultant implant is greater at the center of the scan line than at the edges. The effects of variations in the dopant profile is more complicated than expected, but the dopant profile is significant because the planar dimensions of semiconductor device is ever decreasing. This is of particular importance for ion implantations which are related to device performance, for example, threshold voltage adjustment of metal oxide semiconductor field effect transistors (MOSFETs).
The dopant profile is complex and not as predictable as would be expected from periodic behaviour. It is therefore imperative to devise a method, not only to monitor the dopant distribution for process control but also to enable a better understanding of the mechanisms behind.